[clang] [Clang][Preprocessor] Unify header-name lookahead for import and include (PR #191004)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

I am not sure if we should apply the same policy between the `import` and the `include` cases.

For the `include` case, if the program is ill-formed under https://cplusplus.github.io/CWG/issues/3078.html, it may be reasonable to accept the program as an extension (with an appropriate diagnostic).


https://github.com/llvm/llvm-project/pull/191004